The formation of particles during the high pressure oxidation of silicon in a quartz chamber is due to the solubility of quartz in steam at high temperature and pressure. This solubility phenomenum has received extensive coverage in the literature over the past fifty years (see American Journal of Science, 1977, 277, 1315-1351). The silicon dioxide (quartz, SiO.sub.2) dissolved in the high pressure/temperature steam ambient undergoes gas phase nucleation resulting in spherical SiO.sub.2 particles which are then deposited on the wafer surface by means of complex gas flows within the quartz reaction chamber.
The size of these particles, as determined using a WIS 8500 surface particle counter, vary from 0.3 .mu.m to greater than 2 .mu.m with the majority at about 0.9 to 1.1 lure. The number of particles generated exhibit a large variance. With the commercially available HiPOx.RTM. (a batch high pressure oxidation reactor), produced by GaSonics/IPC, the number of particles will vary from reactor to reactor, typically, for instance, between 1,000 to 1,200 particles at a pressure of 10 atm. With a single wafer HiPOx, the number of particles can be as high as 110,000 at a pressure of about 50 atm. The data and results listed below were carried out using the single wafer HIPOx.
In the single wafer HiPOx, the vessel in which the oxidation occurs is a rectangular quartz chamber having the approximate dimensions of 9.times.8.times.3/8 inches so it will accommodate 150 mm wafers. This quartz chamber is located in a large stainless steel vessel capable of withstanding up to 1,500 psi (100 atmospheres) while still in the elastic deformation mode. The quartz chamber is pressurized by introducing a calculated volume of water (that which is required to provide the processing pressure at a given temperature) which is converted to steam by means of water heaters located prior to the quartz chamber. A zero pressure differential is effected by the introduction of nitrogen into the steel vessel; a link between the quartz chamber and the steel vessel is made through a quartz condensation tube which ensures a zero pressure differential.
In the single wafer HiPOx, under normal processing conditions with the wafer being processed face-down, an average of 110,000 particles are added; under the same conditions with the wafer being processed face-up, about 6,000 particles are added. The difference in particle numbers between face-up and face-down processing can be changed so that both surfaces have 110,000 particles by altering the buoyancy of the system. This is achieved by changing the relative temperatures of the chamber and the water heater which, in turn, causes a change in the complex convective flow system within the chamber.
Previous attempts to minimize particle generation in the commercially available batch reactor have included the use of some silicon carbide parts in the reaction chamber. Other attempts to reduce particle generation have focused on the careful cleaning of all reactor parts. None of these efforts have resulted in a suitable reduction of particles.
U.S. Pat. No. 4,718,975, issued Jan. 12, 1988 to Bowlling et al., addresses a method of reducing particle generation. However, this patent deals with the use of a particle shield to protect the wafer surface but is specifically limited to protection of the wafer during handling and transportation. Further, this patent is limited to reduced particle generation during wafer handling and transportation under vacuum processing conditions, i.e., wafer handling and transportation below atmospheric pressure. Until now, no effective method of particle reduction has existed for use with high pressure systems, i.e., 1 atm. to 100 atm and greater during wafer processing.